This invention relates to a flow control system, in particular for a fuel injector for an internal combustion engine.
In fluid power applications, flow control systems are important constituents that directly define accuracy, reliability, efficiency and cost of the device/installation they belong to. Correspondingly, a flow control system must consume a minimum of energy to control the given fluid power, while being inexpensive, simple, reliable and durable and fulfilling the necessary control accuracy demands. One example of an especially demanding application for a flow control system is a diesel fuel injector. Contemporary diesel fuel injection systems of, for instance, a heavy-duty truck engine are required to deliver high hydraulic power in extraordinarily short bursts with an almost unthinkable accuracy: an instantaneous fluid power in the order of 40 kW can be routinely achieved, its delivery precisely controlled and then fully terminated, all within about 1 ms time slot or less. A fuel injector must keep doing this for up to a billion cycles safely and efficiently while retaining as good controllability as ever over its lifetime. At the same time, being a significant contributor to the overall cost of the engine, the fuel injector is receiving correspondingly high cost reduction attention. It must also be energy efficient, in order for the engine as a whole to attain good fuel economy, whilst affording sufficiently good controllability to allow efficient and clean combustion of the fuel.
Trying to fulfil such a great multitude of conflicting demands, a correspondingly great number of different fuel injectors and their flow control systems have been suggested. However, even the best of the prior art systems have certain drawbacks. For example, the flow control systems that utilize a 3-way solenoid actuator, while benefiting from the advantages this may give in terms of control precision, have a relatively high cost and complexity associated with that actuator, making this approach feasible only for a very few select manufacturers but also carrying their own particular durability and efficiency concerns. Other flow control systems, such as the one disclosed in JP2011202545, are based on a simpler 2-way solenoid actuator and are thus cheaper and may be more durable, but at the same time these tend to have a relatively high control leakage in the hydraulic circuit that amplifies the primary controller's commands and therefore require extremely tight tolerances in order to stay relatively efficient. In addition, this kind of prior art flow control systems/injectors require a compromise to be made between the hydraulic efficiency (the rate of control leakage) and the response time, especially that associated with the closure of the valve/end of injection.
It is desirable to provide a flow control system where the previously mentioned problems are at least partly avoided. According to an aspect of the present invention, a flow control system comprises:                an inlet port for receiving a fluid having a relatively high pressure,        an outlet for letting out said pressurized fluid,        a return port for returning part of said fluid to a volume having a relatively low pressure,        a 2-way control valve comprising a control valve member, a first seat, a first resilient means configured to force said control valve member towards said seat so as to close said control valve, and a first abutment that limits the lift of said control valve member away from said first seat,        
a main valve comprising a main valve member, a second seat, a main control chamber, and an outlet chamber in fluid connection with said inlet port, said main valve member being configured to be forced by pressure in said main control chamber towards said second seat so as to close an opening to said outlet,
a shuttle valve comprising a shuttle valve body, a shuttle control chamber and a third seat, said shuttle valve body being configured to engage with said third seat so as close an opening between said inlet port and said main control chamber;
a connection channel configured to connect said shuttle control chamber with said main control chamber,
wherein said control valve is configured to close and open a connection between said shuttle control chamber and said return port and is biased towards its closed position by said first resilient means, said shuttle valve is biased closed by a second resilient means, said main valve is configured to open and close a connection between said inlet port and said outlet and is biased closed by said second resilient means,
further wherein said shuttle valve is configured such that the pressure in said shuttle control chamber tends to open the shuttle valve whereas the pressure in said main control chamber tends to close the shuttle valve, wherein said main valve is configured such that said pressure in said main control chamber tends to close the main valve whereas a pressure in said outlet chamber tends to open the main valve,
wherein said first seat of said control valve is slidably arranged in said shuttle control chamber and wherein an end stop for said first seat is provided such that the pressure in said shuttle control chamber tends to move said first seat towards said end stop, further wherein said first seat, upon its mechanical contact with said valve member, is able to transmit at least a part of the force of said resilient means onto said shuttle valve body in the opening direction of said shuttle valve.
As mentioned above in the discussion of the prior art, in flow control systems based on the use of a simple two-way control valve coupled to a hydraulic amplification stage to handle the throughput of the high hydraulic power, there is a conflict between the controllability of the flow control system and its hydraulic efficiency. This is because in prior art systems tuned for a quicker and more precise response to the control commands, a higher rate of control flow is required for faster re-pressurization of a hydraulic control chamber and development of a sufficient force to actuate valves. That higher rate of control flow usually entails also a higher rate of control leakage and, as a consequence, worse hydraulic efficiency of the entire system and other undesirable effects such as for example excessive fluid heat-up.
By extending the action of the mechanical resilient means of the control valve also to the shuttle valve, which is a part of the hydraulic amplification unit, a higher rate of leakage can be prevented. That extended action of the resilient means replaces the control flow that is otherwise necessary to initially re-pressurize the control chamber of the shuttle valve upon the flow control system's deactivation command, and thereby reduces the system's control leakage whilst achieving quick control response.
The slidable seat of the control valve may be precision-matched to its guide for limiting the leakage from the shuttle control chamber to the return port that bypasses the actual sealing surface of said seat and the control valve. The slidable seat may be further provided with an additional seating surface at its end stop that limits its movement away from the shuttle valve, such that when at the end stop, that seating surface would form a positive seal with the shuttle control chamber to completely prevent the seat bypass leakage. The shuttle valve may be provided with a differential area exposed to the pressure in the inlet port, in order to improve the force balance occurring on the valve and further shorten the response time to the command for terminating the controlled flow. Another enhancement of the flow control system may be embodied in the form of a poppet attached to the shuttle control valve between its seat and the main control chamber which may also be advantageously configured with a poppet restriction which replaces said fixed restriction between the main control chamber and the shuttle control chamber. By this means, the dynamic behaviour of the shuttle valve may be further improved for greater responsiveness, because the poppet restriction would help creating a positive pressure difference between the shuttle control chamber and the main control chamber and, at the same time, act to increase the effective area for the pressure in the shuttle control chamber and thereby facilitate a faster opening of the shuttle control valve to shorten the response time to the commands for terminating the controlled fluid flow.
According to an aspect of the invention, the flow control system may also include a fuel injection nozzle for additional trimming of the system's flow control characteristics. Said injection nozzle may be connected by its inlet to the outlet of said main valve and may be of a spring-closed type thus providing a faster flow rise and flow drop at correspondingly the flow initiation and termination commands to the flow control system. Said nozzle may be configured to have a needle biased closed by a needle spring, and a needle control chamber, wherein a positive pressure in the needle control chamber biases the needle towards closing the nozzle. The main control chamber of the flow control system may be hydraulically connected to this needle control chamber for a modified control characteristic of the system. Alternatively, the shuttle control chamber may also be hydraulically connected to the needle control chamber, to obtain a slightly slower start of the controlled fluid flow and a slightly faster termination of that flow.
Another embodiment of the present invention may also include a spill valve connected between the high pressure outlet and the volume with a relatively low pressure, for affording the inventive flow control system with an additional possibility of controlling the flow characteristics and providing extra safety features. According to this embodiment, the opening of the spill valve after the termination of the controlled fluid flow through the flow control system would relieve residual pressure between the main control valve and the nozzle and thus prevent possible undesired leakage through the nozzle that might lose its hydraulic tightness due to wear or other damage.
Yet another embodiment may be configured for further improved hydraulic efficiency, by having the spill valve installed between the return port and the volume with a relatively low pressure and the high-pressure outlet connected to the inlet of the spill valve. In this embodiment, the spill valve is closed before the control valve is open to begin the controlled fluid flow. This reduces the leakage out to the volume with a relatively low pressure, and instead directs the pressure relieved by the control valve in the beginning of the system opening into the inlet of the nozzle, so that less hydraulic energy from the outlet chamber of the main control valve would then be used to pressurize the nozzle inlet volume.